1. Field of the Invention
The invention relates in general to a device that accelerates an object or a fluid, and in particular to a device that accelerates an object to high velocity by a helical force field that converts rotational kinetic energy in the device into linear kinetic energy in the object or fluid, and alternatively, that decelerates an object or a fluid from high velocity to low velocity by converting the linear kinetic energy into rotational kinetic energy.
2. Description of the Related Art
There are many different types of accelerating devices. For example, a railgun is a device in which electrical current is made to flow cross-wise through a conductive projectile, causing the projectile to become magnetized. Because magnetic fields and electrical current are repelled by each other, and because this repulsive force always acts in a direction perpendicular to the flow of the electrical current, the projectile is made to accelerate forward in response to this current flow.
Because railguns are powered by electricity, they require heavy and complex systems to store this electrical energy, and to produce and condition their huge electrical power pulses. For example, the University of Texas Center for Electromagnetics is creating an experimental rail gun for the US Marines that will accelerate a 2 kg projectile to 2.5 km/s. The railgun requires a power system that produces a 30 GigaWatt electrical pulse, stores hundreds of megajoules of energy, and weighs many tens of tons.
Railguns operate at extreme current densities. As a comparison, a resistance welder, which uses electrical current to melt and weld material, operates at a fraction of the current density of typical high energy railgun. The high current density required by railguns causes extreme wear on the rail and barrel, and as a result, practical railguns can achieve projectile velocities of no more than about 2.5 km/s. Railguns that do reach greater velocities are typically single-shot, or nearly single-shot.
In a railgun, the accelerating magnetic field is produced by what is essentially a single-turn coil. Generating the required high magnetic-flux density using such a coil requires an extremely high current density, combined with a relatively low voltage. However, concerns over the maximum current carrying capacity of the conductors typically limit a railgun's magnetic flux density to approximately 5 Tesla, which in turn limits a railgun's accelerating force.
Railguns use an arc of plasma to make the electrical contact between the projectile and the rails. Therefore, it is essential that this plasma arc accelerates at the same rate as the projectile. However, with existing railgun technology, it is not possible to control the plasma arc in a repeatable manner when operating at very high velocities and power densities. As a result, the plasma arc typically either lags behind the projectile, or passes it, further limiting the efficiency and maximum velocity that a railgun can attain.
In a coilgun, also known as a “mass driver”, or “co-axial accelerator”, a projectile is made to pass through a series of electromagnetic coils, or solenoids. These solenoids are precisely controlled to turn on, or become magnetic, as the projectile is approaching, and to turn off the instant the projectile passes, allowing the projectile to be pulled forward by the next solenoid in the series.
The magnetic pressure that is applied to an object by a solenoid decreases with the square of the distance between the object and the solenoid's center. Therefore, to get the maximum efficiency out of a coilgun, the projectile must be allowed to approach as closely as possible to the center of each soil (solenoid) before the coil is turned off. However, if the projectile is allowed to pass through the center of the coil before the coil is completely turned off, the magnetic force that was previously accelerating the projectile will now be pulling it back, causing the projectile to slightly decelerate. As the ultimate velocity of the projectile increases, the turn-off time of each coil must decrease for the efficiency of the accelerator to be maintained. However, it is a fundamental characteristic of magnetic coils to create self-generated magnetic fields, which act to keep the coils partially energized (and thus partially magnetized) even when there is no current flowing to them. This characteristic of magnetic coils makes it very difficult to turn them off quickly enough. As a result, the efficiency of the coilgun decreases rapidly with increasing projectile velocity, and coilguns that operate at practical energy density levels are even more limited in their velocity than railguns.
In a conventional (explosive) gun, expanding gas from a chemical explosion pressurized the inside of a barrel behind the projectile. Because the projectile forms a sliding seal between itself and the barrel, it is accelerated by the pressurized gas behind it.
Due to gas dynamics limitations, a chemical-explosive gun cannot accelerate a projectile to a velocity that exceeds the blastwave velocity of the explosive being used. The highest blastwave velocity attainable with a chemical explosive is 2 km/sec. Therefore, even if provided with an infinitely long barrel, a conventional gun cannot accelerate a projectile beyond 2 km/s. Furthermore, the tremendous amounts of ammunition that would be required to operate a conventional gun for extended periods at high rates of fire would make it highly impractical for applications involving continuous operation, such as cutting or drilling.
A light gas gun uses a chemical explosive to produce the energy used to accelerate the projectile. However, a light gas gun circumvents the blastwave velocity limitations of a conventional gun by using its explosive to first accelerate a specific volume of low density gas, or “light gas”, such as hydrogen, which is held in a series of stages behind the projectile. Upon discharge, a sliding piston, driven by the expanding gas from the conventional explosion, compresses the lower density gas in front of it, creating a second blastwave. However, unlike the relatively massive byproducts that make up the conventional explosive's blastwave, the lower mass of the “light” gas allows it to be driven to a much higher velocity by the same amount of energy. As a result, projectiles fired from light gas guns can reach velocities of 8 km/s or more.
Each shot of a light gas gun requires extensive manual preparation. For example, they typically use an exploding metal valve between each stage, which must be replaced after each shot, making continuous firing impractical. Furthermore, because barrel length and piston mass increase rapidly with projectile mass and velocity, light gas guns do not scale well to larger sizes. This characteristic limits the use of light gas guns to highly specialized research applications, within controlled laboratory environments.